The partial pressure method itself, from a logical point of view, is really simple and practical: measuring the amount of gases through their pressure, they are added step by step proportionally to the desired composition. 

In other words, the assumption is a direct  pressure-quantity relationship.

Unfortunately it is not that simple; this is valid only with the Ideal Gas model.

In a nutshell, from the process perspective, to successfully blend  a mixture by partial pressures we must focus on the management of three key points: 
 

Tech Gas Blender has this task: to act as an interpreter, making the amount of gases readable by the pressure property.

Of course there are other key components (outside the process - ex measuring instrument),  which are taken into account in the section Errors in Blending.